This invention relates to a numerical control system for a machine tool and more particularly to an improved apparatus and method for cutting threads in a part on a lathe or similar machine tool.
When using a machine tool such as a turret lathe to cut threads in a piece of cylindrical stock, the cutting tool must proceed at a uniform rate of speed that is synchronized with the spindle speed so that the appropriate number of threads per inch will be cut into the stock. As a numerical example, to machine ten threads per inch, the spindle must make ten revolutions for each inch of cutting tool motion. At a typical spindle speed of 100 to 200 rpm, the time consumed in cutting 10 threads will be approximately 3 to 6 seconds.
A complication arises in that the thread depth is usually such that the thread cannot be cut in one pass of the cutting tool. A thread with a depth of 100 thousandths of an inch, for instance, may be cut in seven passes of the cutting tool. It is therefore necessary that the cutting tool retrace its path exactly, and so, in addition to regulating the spindle speed and rate of travel of the cutting tool it is also necessary to make sure that the tool path falls along the same cutting line on the stock.
One method of synchronizing the angular position of the spindle with the cutting tool motion is to provide a mechanical linkage between the spindle and the drive screw or lead screw on which the cutting tool is mounted so that the position of the spindle will be directly associated with the position of the cutting tool. Thus, the spindle and lead screw will be driven synchronously and the cutting tool will repeatedly fall along the same thread line on the stock being machined. Using this arrangement, the spindle speed can be varied without affecting the accuracy of the system in that when the spindle speed increases, the rate of travel of the cutting tool will also increase proportionately.
Electrical linkages between the spindle and the cutting tool have also been used. In one configuration the spindle generates a specific number of pulses per revolution which are applied as inputs to a stepping motor driving the lead screw so that the advancement of the cutting tool is related to the spindle speed in discrete steps.
Closed loop servo systems are also available for cutting threads. In a typical servo system a commanded position signal is derived from the spindle angular position, is multiplied by a suitable constant to convert it into a position command and is then applied to the closed loop servo driving the lead screw. A resolver monitors the actual angular position of the lead screw and generates a signal proportional thereto. Finally, the commanded position and the actual position are compared in a summing junction and an error signal is generated, closing the loop. A DC tachometer may be attached to the lead screw to generate a signal corresponding to the actual lead screw rotational speed which is compared against the commanded speed resulting in a speed error which is used to close a speed loop within the larger position loop. Through the use of this circuit, the position of the cutting tool is synchronized to the spindle angular position.
In machine tools equipped with a numerical control system, tool repositioning commands are normally read into the system from a paper tape and are used to develop inputs to the spindle drive motor and cutting tool servo loop. Resolvers or encoders are used to monitor the angular position of the spindle and the lead screw, and the resolver outputs are digitized and used as input data to an interpolator which generates a continuous series of motion commands therefrom. These motion commands are then used as inputs to the remainder of the positioning system. The complex circuits required to implement these functions are expensive to design, maintain and calibrate accurately.